NIST crafts molecular solder iron

Portland, Ore. - While molecular-size components such as diodes and resistors built from individual atoms have been demonstrated in laboratories worldwide, their exceedingly small size creates some fundamental problems. One is how to solder these tiny components into circuits. Now the National Institute of Standards and Technology (NIST) is offering a patented solution: a molecular soldering iron for researchers who are assembling electronics one atom at a time.

"We patented this process because we knew of the need for something like a molecular soldering iron from U.S. researchers," said NIST researcher Chris Zangmeister, who said that NIST is "cooperating with universities and companies like Hewlett-Packard" on the project.

The process, co-developed by Zangmeister and fellow NIST researcher Roger van Zee, connects molecular components with copper by selectively coating only the ends that need to be soldered. A layer of molecular components is first assembled with all their ends pointing upward from the substrate. The dangling ends get coated, so that future devices can be soldered into a second layer in a multilayered-chip construction similar to the way silicon chips are built.

NIST and other U.S. and foreign labs have already demonstrated molecular components based on carbon molecules that function as diodes, resistors, wires and even transistors. But their extreme density-more than 10,000x greater than existing electronic components-makes them impossible to solder together into working circuits .

"The question we asked ourselves was whether you could use the molecular components themselves to attract the copper atoms to their ends-and our answer is yes, it can be done, and this is how," said Zangmeister.

The first step was to bond the molecular components permanently to the substrate, in this case gold. That was done by forming a strong covalent chemical bond by virtue of microcontact printing. With a stamp similar to those used on a printing press, the molecules bond with the gold, forming a self-assembled monolayer of organic molecular components.

Once firmly attached to the substrate, the dangling, unconnected ends of the components are selectively coated with copper. In the past, attempts to solder individual elements from such thin films of molecular components gave very low yields, since many of the copper molecules fell "in between" the molecular components, shorting them out. But the NIST microprinting process pretreated only the ends of the molecules, in any desired pattern; the copper sticks to only those components that need to be soldered. "We have been able to increase yields from just a few percent, without our process, to rival that of commercial chip yields," said Zangmeister.

The key, he said, was to functionalize the dangling ends of the components before exposing them to electroless deposition plating baths. By patterning the functionalized molecules (by attaching a carboxylic acid on each end), the researchers were able to demonstrate preferential copper deposition only on the desired components.

"In the future, we hope others will license our process to create multilayer working circuits using molecular components from the bottom up," said Zangmeister.

Zangmeister and van Zee have now turned to characterizing molecular-size electronic components that are compatible with their multilayer, bottom-up circuit-soldering method. The idea is to create a catalog of characterized molecules for all of the common circuit functions.

"We are trying to figure out all the fundamentals of charge transport in molecular-size components," noted Zangmeister. "We want to understand not only how to attach them to surfaces and interconnect them in layers, but also how different types of metal connections between them change their conductance and other electrical properties."